Life Cycle Assessment of Gasoline Blending Options - Environmental

However, different results are obtained for high-octane gasoline (98 RON), where increasing reformer temperatures and pressures increase the reformate...
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Environ. Sci. Technol. 2003, 37, 3724-3732

Life Cycle Assessment of Gasoline Blending Options T E R E S A M . M A T A , * ,† RAYMOND L. SMITH,‡ DOUGLAS M. YOUNG,‡ AND CARLOS A. V. COSTA† Laboratory of Processes, Environment and Energy Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal, and National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268

A life cycle assessment has been done to compare the potential environmental impacts of various gasoline blends that meet octane and vapor pressure specifications. The main blending components of alkylate, cracked gasoline, and reformate have different octane and vapor pressure values as well as different potential environmental impacts. Because the octane and vapor pressure values are nonlinearly related to impacts, the results of this study show that some blends are better for the environment than others. To determine blending component compositions, simulations of a reformer were done at various operating conditions. The reformate products of these simulations had a wide range of octane values and potential environmental impacts. Results of the study indicate that for lowoctane gasoline (95 Research Octane Number), lower reformer temperatures and pressures generally decrease the potential environmental impacts. However, different results are obtained for high-octane gasoline (98 RON), where increasing reformer temperatures and pressures increase the reformate octane values faster than the potential environmental impacts. The higher octane values for reformate allow blends to have less reformate, and therefore high-octane gasoline can have lower potential environmental impacts when the reformer is operated at higher temperatures and pressures. In the blends studied, reformate and cracked gasoline have the highest total impacts, of which photochemical ozone creation is the largest contributor (assuming all impact categories are equally weighted). Alkylate has a much lower total potential environmental impact but does have higher impact values for human toxicity by ingestion, aquatic toxicity, terrestrial toxicity, and acidification. Therefore, depending on environmental priorities, different gasoline blends and operating conditions should be chosen to meet octane and vapor pressure specifications.

Introduction Petroleum refineries are facing the challenge of producing gasoline that contains desirable properties and complies with * Corresponding author phone: 351 225081687; fax: 351 225081449; e-mail: [email protected]. † University of Porto. ‡ U.S. EPA. 3724

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ever-increasing environmental regulations and health restrictions. The impact of gasoline on the environment is directly related to its composition and properties (1, 2). Gasoline is a complex mixture of hydrocarbons obtained by the combination of two or more refinery products (with some additives). Gasoline blends can vary widely in composition; even those having the same octane number can be quite different. The various properties of gasoline depend on the types and relative proportions of each of their constituents. In gasoline blending, important specifications such as the Research Octane Number (RON) and the Reid Vapor Pressure (RVP) have to be met simultaneously. This is not always a straightforward process since the RON and the RVP are not linearly related. Blending parameters are influenced by the nature of the blending components and their hydrocarbon composition. Several procedures are available to predict blending parameters (3, 4). Gasoline blending is carried out by simultaneously pumping all the components of a gasoline blend into a pipeline that leads to storage. Butane, reformate, alkylate, and cracked gasoline are common gasoline blending components (5, 6). Reformate and cracked gasoline are rich in aromatics and isoparaffins, which represent the principal sources of octane in gasoline. Alkylate is rich in isoparaffins and has a relatively high octane number, serving to dilute the total aromatics content. Alkylate promises to become even more important as environmental concerns lead to the reduction of aromatics, olefins, and (in the United States) additives such as methyl tert-butyl ether in gasoline. Besides cracked gasoline, reformate, and alkylate, a certain amount of butane is dissolved in gasoline, which facilitates vaporization at low temperatures, especially during the winter. The subject of gasoline blending brings into focus most of the upstream operations in a refinery. Catalytic reforming is used as a means of upgrading the octane number of gasoline. It is usually applied to a straight-run naphtha rich in paraffinic and naphthenic hydrocarbons combined with recycle gas that contains C1 to C5 gases and hydrogen (7). This mixture is heated to the desired temperature of 460540 °C and then passed through fixed-bed catalytic reactors at pressures of 500-3000 kPa. Normally, several reactors are used in series, and interstage heaters are located between reactors to compensate for the endothermic reactions taking place. There are several reactions taking place in catalytic reforming: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. The catalyst used is principally platinum or platinum-rhenium on an alumina base (8). Hydrogen is produced as a byproduct in large quantities as a consequence of the dehydrogenation reaction. A certain amount of this hydrogen is recycled through the reactors where the reforming takes place, preventing carbon from being deposited on the catalyst and thus extending its operating life. The remaining hydrogen can be used in other refinery processes such as hydrotreating, hydrocracking, and olefins saturation. In this study, simulations were performed for the reforming process, which is the most variable in terms of its product: the reformate RON ranges between 80 and 110 depending on the reformer operating conditions, increasing with the temperature and pressure (9). Process simulation is one way of coping with all the possible variables in this operation, and the calculated values are estimates of the real process, allowing different aspects to be easily analyzed. 10.1021/es034024s CCC: $25.00

 2003 American Chemical Society Published on Web 07/08/2003

A frequent problem encountered with using naphtha as a feedstock is the unpredictable nature of the feed stream composition. In the present study, the naphtha feed composition as determined by Moljord et al. (10) through gas chromatography analysis was used for simulating the reforming process. This model feed is characterized by naphthenes (alkyl cyclohexanes and alkyl cyclopentanes), paraffins (normal and iso paraffins), and aromatics lumped in terms of each carbon number from C5 to C9 (10). Alkylation is the process of combining paraffins with olefins to produce highly branched isoparaffins boiling in the gasoline range. Alkylate is a complex mixture of hydrocarbons containing about 20% C5 to C7 compounds, 60-65% C8 compounds, and 15-20% compounds of higher molecular weight (11). Although several authors have reported their findings based on experimental work (11-13), the alkylation process includes several complex reactions, whose kinetics have not been identified yet. For this reason, and the fact that the alkylate composition is relatively constant, the alkylation process is not simulated in this study, and an alkylate composition given in the literature is used (13). The fluid catalytic cracking (FCC) process is also not simulated in this study; it is assumed to have a composition given in the literature (14-17). The FCC process converts gas oil into gasoline and distillate fuel products. The FCC consists of a riser that cracks the feed into lighter fuel products and a regenerator that burns the coke that deposits on the catalyst (18). Considerable work has been done trying to understand the FCC reaction mechanism (19, 20). The reported works are based on a lump model, which consists of lumping molecules in distillation cuts and considering pseudochemical reactions between these lumps. Within these processes and in refineries in general, reducing volatile organic compound (VOC) evaporative and leak emissions has assumed a high priority. At the political level, the EU Directives 98/70/EC (21) and 2000/71/EC (22) established some gasoline specifications that not only ensure acceptable engine performance but also act to reduce the environmental impact of gasoline production. For example, according to the EU Directive 2000/71/EC, aromatics content and benzene in particular must be reduced. Also, it specifies an RVP of 60 kPa maximum to be in effect during the summer. Regulations on gasoline volatility (RVP) and VOC emissions vary geographically and seasonally and are detailed in the EU Directive 94/63/EC (23), which defines some measures to control VOC emissions. Gasoline refining, storage, handling, transportation, and marketing involve many distinct operations, each of which represents a potential source of evaporation losses, as equipment leaks result in fugitive emissions. Substances emitted to the atmosphere from gasoline activities are the cause of many current and potential environmental problems. It is necessary to have quantitative information on these emissions and their sources to evaluate the potential environmental impacts (PEI) and implications of different strategies and to set explicit objectives and constraints for environmental improvement (24, 25). Several LCA studies have been proposed in the literature to compare alternative automobile fuels, addressing different concerns from this study and with different levels of sophistication (26-31). For example, DeLuchi (26) estimates emissions of VOC, CO, NOx, SO2, and particulate matter, from gasoline production and marketing, comparing three existing automobile fuels, reformulated gasoline, conventional gasoline, and low-sulfur diesel fuel, but does not estimate their potential environmental impacts. Eriksson et al. (27) estimate energy and pollutant emissions from the production and combustion of reformulated gasoline and diesel. In three life cycles of automobile fuels including regular gasoline, gasoline with MTBE, and diesel, Furuholt (28) accounts for energy consumption and CO2, NOx, SO2, and VOC emissions and

evaluates their potential environmental impacts. Lave et al. (29) examine the economic and environmental implications of the fuels and propulsion technologies that will be available in the next years. A life-cycle perspective is used to analyze fossil fuels, compressed natural gas, ethanol from biomass, and electricity for internal combustion engines and electric vehicles. Maclean and Lave (30) analyze alternative fuelpowertrain options for internal combustion engine automobiles and estimate fuel/engine efficiency, energy use, pollutant discharges, and greenhouse gas emissions. A model for the evaluation of fuel cycles is GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation), developed by Wang (31), which considers air emissions of greenhouse gases and criteria pollutants from crude oil extraction to vehicle use. In this study a life cycle assessment has been done to compare the potential environmental impacts due to evaporative and leak emissions from several gasoline blends composed of reformate, alkylate, cracked gasoline and butane, while using the RON and RVP criteria as constraints on the various acceptable gasoline blends. Other constraints not considered in this work include sulfur, aromatic, and olefin limits in blended gasoline. The gasoline blends to be compared in this study are quite similar products; therefore, it was decided to include in the system boundaries the life cycle stages from petroleum refining to vehicle refueling and to exclude elements such as oil drilling and emissions from vehicles. Therefore, this study accounts for the gasoline VOC emissions from petroleum refining to vehicle refueling and evaluates the potential environmental impacts using the Waste Reduction (WAR) algorithm. The EPA’s WAR algorithm (32), described in the Supporting Information, is used to evaluate the potential environmental impacts, including eight impact categories: human toxicity by ingestion and by dermal/inhalation routes, terrestrial toxicity, aquatic toxicity, photochemical oxidation, acidification, global warming, and ozone depletion.

Methodology and Assumptions When studying a large process it is not practical to measure emissions from all of the individual sources. In practice, atmospheric emissions are estimated on the basis of measurements made at selected or representative samples of the main sources and source types. The methods for estimating mass emissions from process-equipment leaks range from the use of average emission factors to comprehensive field measurement techniques. These methods have evolved from a number of studies of the organic chemical and petroleum refining industries by the U.S. EPA (17, 33). Average emission factors are listed in AP-42 (34). The Protocol for Equipment Leak Emission Estimates (35) describes testing procedures, such as screening or bagging (or both) involved in the development of emission factors. Fugitive emissions of gasoline are influenced by several factors: volatility (measured as Reid Vapor Pressure), the technology for loading tank trucks and tanks (splash loading, submerged loading, vapor balance, etc.), and storage tank characteristics (color and design). Methods for estimating emissions from parts of the gasoline marketing system, gasoline trucks in transit, fuel delivery to outlets, and storage tank breathing are provided by U.S. EPA (36) and AP-42 (34). System Boundary and Emission Sources. Emissions of hydrocarbons to the atmosphere occur in nearly every element of the gasoline production and distribution chain. After its production, gasoline distribution starts at the refinery or at the border terminal where it is loaded into rail cars, barges, coastal tankers, or pipelines, for delivery to market depots or service stations. At service stations, gasoline is transferred into underground storage tanks and subsequently dispensed into vehicle fuel tanks. VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. System boundary definition for this study including the gasoline life cycle stages. Processes within the boundary are considered in terms of emissions, while dashed arrows crossing the boundary are not. Figure 1 shows the system boundary, providing information on the activities included or excluded from this study. Several assumptions were made to restrict the system boundary, keeping in mind the objectives of this study. For example, since the study objective is to compare various gasoline blends of 95 and 98 RON with a RVP upper limit of 60kPa, the inventory analysis is focused on estimates of the fugitive emissions due to evaporation and leaks of the different gasoline blends from the following: (1) gasoline production in the refinery, including alkylation, catalytic reforming, catalytic cracking, and gas processing; (2) distribution, including loading and transportation of gasoline using rail tank cars and tank trucks, tanker ships and barges, pipelines, and return of empty tanks to bulk plants; (3) product storage and handling, including transfer of gasoline from tank trucks to storage tanks at the service stations, evaporation of gasoline from the storage tanks and from the lines going to the pumps during the transfer of gasoline; (4) vehicle refueling, including displacement of vapors from vehicle tanks during refueling, spillage, and subsequent evaporation. Operations that are assumed to be the same in all cases, and are therefore excluded from the inventory, are the following: (1) crude oil extraction, handling, storage, treating, and distillation; (2) auxiliary facilities, e.g., combustion, process furnaces, sulfur recovery plants, wastewater treatment, incineration of wastes, incineration of sludge from water treatment, flaring in refinery, landfills, cooling towers, and vapor recovery; (3) generation and distribution of energy and construction of distribution mechanisms and facilities; and (4) emissions from vehicles (resting and running losses, exhaust, diurnal, and hot soak emissions). 3726

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Methodology for the Study. The proposed methodology is based on the analysis of the refinery process and determination of the amount of gasoline produced and distributed per year. It also includes an estimation of average emission factors for the gasoline flows from the refinery to car filling stations and the various loading and unloading techniques. Then, the determination of the hydrocarbon emissions is based on the estimation of the gasoline composition. Finally, the evaluation of the potential environmental impacts was done using the WAR algorithm. This study accounts for the hydrocarbon emissions from four processes in the refinery: the catalytic reforming of straight-run naphtha into aromatics and iso-paraffins, the alkylation of propylene and butylene with isobutane to form iso-paraffins, the catalytic cracking of straight-run heavy gas oils, and light-ends recovery and processing at the gas plant. Figure 2 shows a diagram of the reforming process, which consists of a plug flow reactor, a flash vessel, a distillation tower, a valve, two heat exchangers, and a recycle loop for hydrogen recycle with an additional compressor and a heat exchanger. Simulations of the catalytic reforming process were performed using the simulator PRO/II of Simulation Sciences Inc. For the simulations, a 20% recycle of hydrogen was used, and it was assumed that the reactor system is represented by one isothermal reactor, which converts around 8 × 108 kg/yr of naphtha, composed of paraffins, five- and six-membered ring naphthenes, and aromatics. The simulations were limited to reactor operating conditions of 460, 500, and 540 °C and 500, 1500, and 3000 kPa. The model of the process assumes a total of 91 reactions and 43 chemical species from C4 to C9. The reactions and kinetics used for the reforming process simulation are described in

FIGURE 2. Diagram of the catalytic reforming process simulated for this study.

TABLE 1. Average Hydrocarbon Emission Factors

stage 1 stage 2 stage 3 stage 4 stage 5 stage 6 stage 7 stage 8 stage 9

stages

units

emission factors

refinery (combined factor for valves, flanges, pump seals, compressors, other) loading of gasoline in tank trucks and tank cars transit losses from rail tank cars and trucks gasoline loading losses from ships and barges transit losses from ships and barges emissions from filling underground gasoline storage tanks at service stations underground tank breathing losses due to withdraw of gasoline from the tank marketing terminal and service station emissions not otherwise considered motor vehicle refueling emissions and spillage loss of dispensed gasoline

kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3

6.7100 0.2362 0.0084 0.1922 0.0065 0.7667 0.1200 3.3550 0.5970

Padmavathi and Chaudhuri (7). The content of the reformate varies from 39.89% to 24.68% paraffins, 9.45% to 0.01% naphthenes, and 50.66% to 75.31% aromatics. These values correspond to the operating limits of the reformer. The composition of alkylate described by Simpson et al. (13) includes 17 chemical species from C5 to C9 alkane isomers. Nagpal et al. (15, 16) list a typical fluidized catalytic cracker (FCC) naphtha fraction. Tables 7-20 of the U.S. EPA (17) show the contribution of each component of the FCC effluent to air emissions. Hamilton (14) gives a typical composition of gasoline and components in the olefins group. It was considered for cracked gasoline that there are 48 chemical species from C5 to C9, including 21.6% paraffins, 6.7% naphthenes, 54.6% aromatics, and 17.1% olefins. The methodology for estimating emissions is based on average emission factors, combined with information on pumped volume of gasoline produced per year by a refinery. This pumped volume of gasoline depends on the production rate of reformate, which depends on the reactor operating conditions described above. The estimated emissions are described by

emission ) (average emission factor) × (pumped volume of gasoline) (1) Once the amounts of the blending components (e.g., reformate, etc.) have been normalized by the total amount of gasoline, the results can be scaled up to 6 × 106 m3/year (using a specific gravity of 0.67 at 15.6 °C). The average emission factor can be estimated based on type, efficiency, and extent of emission control measures, applied in distinguished sectors. It requires knowledge of specific parameters of the gasoline distribution system and basic physical and chemical parameters of the distributed gasoline. Table 1 summarizes the average emission factors used in this study to estimate the hydrocarbon emissions from refining, distribution, and refueling. Stage 1, for the refinery, has been approximated as having 1.0% emissions (37), while stage 8 (the marketing terminal and service station emissions not otherwise considered) has been approximated as having 0.5% emissions, which is within the range calculated from the European Environment Agency (38). The other stages have

been approximated through emission factors (17, 33-36). Stage 1 includes the gasoline losses from valves, flanges, pump seals, compressors, etc. These emissions (55.95 wt % of the total gasoline losses) are attributable to the evaporation of leaked or spilled gasoline liquids, where valves are usually the largest source (37). The composition of these emissions will be equal to that of the leaking gasoline, as described in the Protocol for Equipment Leak Emission Estimates (35), which suggests eq 2 to estimate emissions from equipment of a specific VOC in a mixture of several chemicals (33, 35)

Ex ) ETOC ×

WPx WPTOC

(2)

where Ex is the mass emission rate of organic chemical x in kg/h, ETOC is the TOC (total organic carbon) mass emission rate in kg/h, and WPx and WPTOC are the weight percent concentrations in the equipment of chemical x and TOC, respectively. Stages 2-7 include emissions from loading and transit of rail tank cars, tank trucks, marine vessels, and underground storage tanks. Loading losses (10.97 wt % of the total gasoline losses) occur when gasoline vapors in empty tanks are displaced to the atmosphere by the liquid being loaded into the tanks. The composition of these vapors will be that of the residual product from previous loads that evaporated (34). Breathing losses (representing 0.12 wt % of total losses) occur during transit due to displacement of vapors from filled tanks. The composition of these emissions is that of the vapors in equilibrium with the liquid inside the tanks and can be estimated using Raoult’s law. Because these emissions represent a small amount of the total gasoline losses, i.e., 0.12%, it was assumed in this study that the emissions have the same composition as the gasoline. Stage 8 includes the marketing terminal and service station emissions from valves, flanges, pump seals, compressors, etc. These emissions, 27.98 wt % of the total losses, are attributable to the evaporation of leaked or spilled gasoline liquids. Thus the composition of these emissions will equal that of the leaking material and can be estimated using eq 2. Stage 9 includes the motor vehicle refueling emissions (4.31 wt % of total losses) and spillage loss of dispensed VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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gasoline (0.67 wt % of total losses). These come from vapors displaced from the automobile tank by dispensed gasoline and from spillage, respectively. The composition of the refueling emissions is that of the vapors in equilibrium with the liquid inside the vehicle tank and can be estimated using Raoult’s law. Because these emissions represent a small amount of the total gasoline losses, i.e., 4.31%, it was assumed in this study that they have the same composition as the gasoline.

Results and Discussion Several gasoline blends were studied and compared in terms of the PEI generated due to hydrocarbon emissions. Detailed simulations of the reforming process were performed since it is the most variable in terms of products and octane number. Using temperatures of 460, 500, and 540 °C and pressures of 500, 1500, and 3000 kPa in the reformer, the obtained reformate has a RON that ranges between 89.7 and 108.4 with an RVP that ranges between 13.0 and 23.9 kPa. Thus, depending on the reformer operating conditions, reformate is the gasoline blending component that can contribute the most to increase the RON of a blend. The methods for calculating the RON and RVP are described by Nelson (39) and by Owen and Coley (4). The RON was calculated using the octane number of each component, and the RVP was calculated by relating it to the true vapor pressure (TVP). TVP was calculated as described in Appendix A of Reid et al. (40). The RON and RVP for the other components are listed in Leffler (3): alkylate has a RON of 97.3 and a RVP of 31.7 kPa, cracked gasoline has a RON of 92.3 and a RVP of 30.3 kPa, and butane has a RON of 93.0 and a RVP of 489.5 kPa. These values were assumed to be constant for this study even though the alkylate and cracked gasoline components could vary slightly in composition. The RVP of butane is approximately 20-40 times larger than the RVP of reformate and more than 15 times the RVP of alkylate and cracked gasoline. Therefore, small variations (e.g. 1%) in the amount of butane can make the RVP of the mixture jump above the upper limit of 60 kPa. For this reason, to maintain the RVP requirements, in this study only 6% butane is used in the gasoline blends. Note that butane is being used here at a quantity of 6 wt % to represent itself and a number of additives that add to RON and RVP. The intention is not to imply that all gasoline blends will have 6% butane, but that they will have additives and light ends (constrained by the RVP limit) that significantly effect RON and RVP. Gasoline Blending Possibilities. Maintaining the requirements of 95 and 98 RON and an RVP maximum limit of 60 kPa, several gasoline blends are analyzed in this study. These gasoline blends were obtained by combining a proportion of each gasoline blending component (butane, reformate, alkylate, and cracked gasoline) in order to meet the desired RON and RVP. The proportion of each gasoline blending component is influenced by its individual characteristics (RON and RVP). In this study the RON and RVP of butane, alkylate, and cracked gasoline are fixed and vary for reformate depending on the reformer operating conditions. Generally, for higher temperature and pressure in the reformer, the RON of reformate is higher. This is explained by higher aromatic and iso-paraffin content, which have higher octane numbers. The several gasoline blends analyzed in this study are represented in the triangular graphs of Figure 3. Lines (i)(ix) in Figure 3a refer to blends of 95 RON gasoline and lines (x)-(xiv) in Figure 3b refer to blends of 98 RON gasoline. In these blends the reformer is operated at various temperatures and pressures to produce reformate having different RON. For all these gasoline blends butane is 6 wt %. These triangular graphs show only the three main gasoline blending com3728

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FIGURE 3. Blending lines for (a) 95 and (b) 98 RON gasoline. The amount of butane is constant at 6 wt %, so the remaining components are normalized on a percent basis without butane. (a) 95 RON gasoline: (i) T ) 460 °C, P ) 500 kPa, RONREF ) 89.7; (ii) T ) 460 °C, P ) 1500 kPa, RONREF ) 93.5; (iii) T ) 460 °C, P ) 3000 kPa, RONREF ) 94.9; (iv) T ) 500 °C, P ) 500 kPa, RONREF ) 95.2; (v) T ) 500 °C, P ) 1500 kPa, RONREF ) 98.6; (vi) T ) 500 °C, P ) 3000 kPa, RONREF ) 101.1; (vii) T ) 540 °C, P ) 500 kPa, RONREF ) 101.4; (viii) T ) 540 °C, P ) 1500 kPa, RONREF ) 105.5; (ix) T ) 540 °C, P ) 3000 kPa, RONREF ) 108.4. (b) 98 RON gasoline: (x) T ) 500 °C, P ) 1500 kPa, RONREF ) 98.6; (xi) T ) 500 °C, P ) 3000 kPa, RONREF ) 101.1; (xii) T ) 540 °C, P ) 500 kPa, RONREF ) 101.4; (xiii) T ) 540 °C, P ) 1500 kPa, RONREF ) 105.5; (xiv) T ) 540 °C, P ) 3000 kPa, RONREF ) 108.4. ponents (reformate, alkylate and cracked gasoline), since butane is 6% for all the gasoline blends, the other three components (summing 94%) have been normalized to 100%. The slopes of the lines depend on the RON of the gasoline blending components, in particular on the RON of reformate which varies with the reformer operating conditions, since the RON of alkylate and cracked gasoline are fixed. In Figure 3a three distinct zones can be distinguished, which are delineated by the dotted lines. The first zone includes the line (i), referring to gasoline blends with reformate having 89.7 RON, which is smaller than those of cracked gasoline (92.3 RON) and alkylate (97.3 RON). In fact, the distinctive feature of all of the gasoline blends in this upper zone is that the reformate component has a RON less than that of cracked gasoline (and alkylate). Thus, in this first zone, the gasoline blends are rich in alkylate, having less cracked gasoline and reformate. The second zone (lower right of Figure 3a) includes the lines (ii), (iii), and (iv), with similar slopes, referring to gasoline blends with reformate having 93.5, 94.9, and 95.2 RON, respectively. The distinctive feature of the gasoline blends in this zone is that the RON of reformate is larger than that of cracked gasoline (92.3) and smaller than

that of alkylate (97.3). In this second zone, since the RON of reformate is close to 95, the 95 RON gasoline blends are obtained by adjusting the proportions of each gasoline blending component. For example, in this zone the percentage of reformate can vary widely, alkylate is added when there is a need to increase the RON, and cracked gasoline is added to decrease it, with the purpose of obtaining a 95 RON gasoline. Finally, the third zone (lower left of Figure 3a) includes the lines (v), (vi), (vii), (viii), and (ix), referring to gasoline blends with reformate having 98.6, 101.1, 101.4, 105.5, and 108.4 RON, respectively. In this zone all of the reformate RON values are greater than the RON for alkylate (and cracked gasoline). As a result, less reformate is needed to satisfy the 95 RON specification, and the gasoline blends are rich in cracked gasoline. In Figure 3b there are no separate zones. The lines (x)(xiv) (shown in Figure 3b) refer to gasoline blends with reformate having 98.6, 101.1, 101.4, 105.5, and 108.4 RON, respectively. Note that the RON of reformate here is higher than those of alkylate (and cracked gasoline), which is the distinctive feature of Figure 3b, similar to the third (lower left) zone of Figure 3a. Depending on the RON of the reformate, more or less reformate is needed to satisfy the 98 RON specification. Line (x) represents few possibilities for gasoline blends because the reformate, obtained at the reformer conditions of 500 °C and 1500 kPa, has a RON of 98.6, which is close to that of the desired specification (98 RON), and the RON of alkylate and cracked gasoline are lower than 98. Also, for this reason, these gasoline blends are richer in reformate. In Figure 3b there are no gasoline blends with reformate obtained at 460 °C, because the RON of reformate is lower than that of the desired gasoline (98 RON), which would make a blend infeasible using these components. Some recommendations for gasoline blenders can be presented here: when formulating 95 RON gasoline, with reformate having a RON lower than 95, the gasoline blends must be rich in alkylate (or other octane enhancing additives). On the contrary, if the RON of reformate is higher than 97.3, the gasoline blends must be rich in cracked gasoline (i.e., low octane components). When formulating 98 RON gasoline it is important to use reformate with higher RON, obtained at higher reformer temperatures (500-540 °C). A reformate obtained at lower reformer temperature (460 °C) has a RON lower than 98, as do alkylate and cracked gasoline, which would make a 98 RON gasoline infeasible (without resorting to larger amounts of additives that enhance octane). Potential Environmental Impacts of the Gasoline Blends. The potential environmental impacts of the gasoline blends represented in Figure 3a,b have been evaluated using the WAR algorithm (32). The graphs of Figures 4 and 5 show the PEI categories of respectively the 95 and 98 RON gasoline blends described in Figure 3. These gasoline blends have different percentages of reformate, alkylate and cracked gasoline, and a fixed percentage of butane (that is 6 wt % in all the cases). To compare how the PEI changes depending on the percentage of each gasoline component, three distinct gasoline blends have been chosen for further comparison. These three gasoline blends have different percentages of reformate (a small, medium and large reformate content) corresponding to the three bars represented from left to right in the graphs of Figures 4 and 5. Eight PEI categories have been evaluated including terrestrial toxicity potential (TTP), human toxicity potential by ingestion (HTPI), human toxicity potential by exposure (HTPE), aquatic toxicity potential (ATP), global warming potential (GWP), ozone depletion potential (ODP), acidification potential (AP), and photochemical ozone creation (or smog) potential (POCP). Weighting factors can be used to compare PEI categories. The weighting factors represent the relative or site-specific concerns of the user.

FIGURE 4. Potential environmental impacts of 95 RON gasoline blends for three distinct percentages of reformate (small, medium, and large as described in the Supporting Information) versus the reformer pressure of 500, 1500, and 3000 kPa and temperature: (a) 460 °C, (b) 500 °C, and (c) 540 °C. Since in this study there is no specific site in mind, the weighting factors for all the categories have been assigned equivalent values of unity. (Note that although TTP and HTPI are distinct potential environmental impact categories, they have the same magnitude in the WAR database.) An obvious observation of the analysis of Figures 4 and 5 is that photochemical ozone creation potential is the PEI category with the largest and most variable values followed by the aquatic toxicity potential, the terrestrial toxicity potential, and the human toxicity potential by ingestion, respectively. Global warming, ozone depletion, and human toxicity by exposure potentials have negligible PEI values. An important note here is that equal weighting factors have been used for these calculations, whereas other weighting schemes could give different results. Also, it should be pointed out that these results are partially dependent on where the system boundaries were drawn. These categories (or others not considered by the WAR algorithm) may have significant impacts in the areas neglected by this study. However, as described earlier, the boundaries for this study were chosen to eliminate stages of the life cycle that are similar for every gasoline blend. Although not shown here (2), it is important to note that reformate followed by cracked gasoline have the largest VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Potential environmental impacts of 98 RON gasoline blends for three distinct percentages of reformate (small, medium, and large as described in the Supporting Information) versus the reformer pressure of 500, 1500, and 3000 kPa and temperature: (a) 500 °C and (b) 540 °C. contributions to the photochemical ozone creation. Alkylate contributes mostly to the aquatic toxicity potential, terrestrial toxicity potential, and human toxicity potential by ingestion, followed by cracked gasoline and then by reformate. The contribution of alkylate to the acidification potential is the largest, followed by cracked gasoline, while the contribution of reformate is negligible (2). Also, the contributions of gasoline blends to global warming, ozone depletion, and human toxicity by exposure are negligible (2). Note here that this is the result of equal weighting factors and of where system boundaries were drawn (e.g., burning gasoline obviously creates global warming potential but is not within the boundaries of the study). The general associations between impact categories and blending components (Mata et al. (2)) are as follows: POCP increases with cracked gasoline and reformate compositions; AP increases with alkylate and cracked gasoline compositions; and ATP, HTPI, and TTP increase in the order of reformate, cracked gasoline, and alkylate (i.e., alkylate has the highest PEI scores in these categories). In looking at Figures 4 and 5 it is worth noting how the various impact categories are effected by changes in blending compositions. In particular, by changing the reformer operating conditions, the reformate composition changes, which leads to different blends to achieve RON and RVP. In comparing Figure 4a to the other parts of Figures 4 and 5 one can see that there is a qualitative difference in operating the reformer at 460 °C and 500 kPa. The POCP decreases with an increase in reformate composition. However, the cracked gasoline composition is decreasing as reformate increases, and the result is a reduction in POCP. In the rest of Figures 4 and 5 either reformate and cracked gasoline compositions are increasing together or the increase in reformate composition dominates so that POCP increases. It can be concluded from the analysis of the graphs of Figures 4 and 5 that gasoline formulated with smaller amounts of reformate and cracked gasoline (and more alkylate) leads to 3730

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FIGURE 6. Contour plots for different values of (a) total potential environmental impact and (b) photochemical ozone creation potential, at various reformer pressures and temperatures. lower photochemical ozone creation values. Gasoline blends with large amounts of alkylate, less cracked gasoline, and less reformate lead to higher aquatic toxicity, terrestrial toxicity, human toxicity by ingestion, and acidification values. Details on these compositions and their effects on the various impact categories (including detailed descriptions of Figures 4 and 5) are given in the Supporting Information. Total PEI and POCP Contour Plots for Gasoline Blends. In Figure 6 the total PEI and POCP are analyzed using triangular graphs. These triangular graphs represent the contour plots for fixed values of total PEI (Figure 6a) and POCP (Figure 6b) for various percentages of reformate, alkylate, and cracked gasoline and a fixed percentage of butane (that is 6 wt % in all the cases). The POCP is represented by itself in Figure 6 because it is the category, in our uniform weighting, that contributes the most to the total PEI. Note that if a different set of weighting factors had been chosen, the results could have possibly been different (see, for example, Mata et al. (2), which describes a sensitivity analysis of weighting factors for the blending components). Figure 6a shows that the total PEI of gasoline can vary between 6 × 107 and 10 × 107 impact/yr. Figure 6b shows that the POCP can vary between 1 × 107 and 6 × 107 impact/yr. On the figures, the three types of contour lines (solid, dashed, and dotted) represent different reformer operating conditions. The remaining operating conditions have not been represented since their slopes are similar to those shown. Figure 6 shows that reformate obtained at higher reforming temperature and pressure has a larger PEI, which will

contribute to increase the total PEI of gasoline. The total PEI increases as more cracked gasoline and reformate are added to gasoline. However, a lower reformate content does not necessarily mean a lower PEI. For example, a gasoline with 20% reformate has a total PEI varying between 7 × 107 and 10 × 107 impact/yr and a POCP varying between 2 × 107 and 6 × 107 impact/yr, depending on the percentage of cracked gasoline and alkylate blended in gasoline. For a gasoline with 50% reformate the total PEI varies from 8 × 107 to 10 × 107 impact/yr and the POCP from 4 × 107 to 6 × 107 impact/yr. Thus, a gasoline blend with a low reformate content could contain a large amount of cracked gasoline and therefore have a relatively high PEI value. Total PEI and POCP of 95 and 98 RON Gasoline. Considering the triangular composition diagrams of Figures 3 and 6, one can visualize how a blend composition, and therefore reformer operating conditions, effect PEI values. While Figure 6 shows total PEI and POCP, similar diagrams could be developed for each impact category. Thus, one result of this work is the utility of such diagrams for analyzing the potential impacts of gasoline and other blended products. Comparing the graphs of Figures 3 and 6 it can be observed that line (i) corresponds to the 95 RON gasoline blends with the lowest total PEI and POCP. The total PEI of the blends on line (i) can vary between 7 × 107 and 8 × 107 impact/yr and the POCP can vary between 2 × 107 and 4 × 107 impact/ yr. For the gasoline blends of the second zone of Figure 3a, lines (ii), (iii), and (iv), the total PEI and POCP increase as more reformate is added, and the variation of the reformate percentages is larger than those of the other blending components. Blends of line (ii) have lower PEI than those of lines (iii) and (iv). The total PEI of the blends on line (ii) can vary between 8 × 107 and 9 × 107 impact/yr and the POCP between 3 × 107 and 4 × 107 impact/yr. For the blends of line (iii) and (iv) the total PEI can vary between 8 × 107 and 10 × 107 impact/yr and the POCP between 3 × 107 and 7 × 107 impact/yr. Generally, more alkylate decreases the total PEI while cracked gasoline and reformate increase the total PEI. For the gasoline blends in the third zone, lines (v)-(ix), the total PEI and POCP increase as more cracked gasoline and reformate are added and decrease as more alkylate is added. For the blends in lines (v)-(ix) the total PEI can vary between 8 × 107 and 10 × 107 impact/yr and the POCP between 3 × 107 and 6 × 107 impact/yr. In the third zone the variation of the reformate percentages is smaller than those of the other blending components. The PEI behavior of 98 RON gasoline is similar to that of the 95 RON gasoline blends in the third zone, with differences in the range of the PEI variation. For the 95 RON gasoline blends in the third zone, the total PEI is higher than 8 × 107 impact/yr and the POCP is higher than 3 × 107 impact/yr, while for the 98 RON gasoline blends the total PEI and POCP can be lower than that. For example, the total PEI of the blends in line (xiv) can vary between 6 × 107 and 10 × 107 impact/yr and the POCP can vary between 1 × 107 and 6 × 107 impact/yr. For more than 70% alkylate in the blend and higher reformer temperature and pressures the total PEI of 98 RON gasoline can be lower than 7 × 107 impact/yr and the POCP can be lower than 2 × 107 impact/yr. In this case the impacts are lower than the lowest total PEI blend of the 95 RON gasoline blends, i.e., line (i). For line (x), which has the lowest reformer operating conditions for 98 RON gasoline, the total PEI is higher than 9 × 107 impact/yr and the POCP is higher than 5 × 107 impact/yr. This study shows that PEI values vary due to the various compositions of the gasoline blends and with the reformer temperature and pressure. In particular, higher reformer temperatures and pressures lead to higher RON and PEI values of reformate, which will increase the RON and PEI of

95 RON gasoline. Thus, a reformer operating at lower temperature and pressure generates much lower photochemical ozone creation values. So, when formulating 95 RON gasoline, relatively less reformate (and more alkylate) in the blend generally decreases the potential environmental effects. However, when formulating 98 RON gasoline, larger reformer temperature and pressure can lead to lower potential environmental impacts, because the gasoline blends have less reformate, allowing one to use more alkylate and less cracked gasoline. For the lower RON gasoline, one has to replace the high octane reformate with cracked gasoline to satisfy the RON specification, so that zone 3 blends (Figure 3a) are obtained with their corresponding high PEI values (Figure 6). On the other hand, for the higher RON gasoline, the reformate generated at high temperature and pressure can be replaced with alkylate (Figure 3b), which leads to lower PEI values (Figure 6). Generally, for 95 RON gasoline, higher reforming operating conditions tend to increase the total PEI and POCP. For 98 RON gasoline, higher reforming operating conditions allow a gasoline blender to decrease the PEI by increasing the percentage of alkylate and decreasing the percentage of cracked gasoline. Gasoline blends of 98 RON have lower total PEI values when formulated with more alkylate and less cracked gasoline and reformate. Thus, the most environmentally friendly reformer operating conditions depend on the characteristics (e.g., RON) of the blend as well as the reformate PEI values. With the above information on the PEI variations depending on the gasoline composition, an engineer could devise refinery operating policies that obtain desired products while minimizing the environmental effects.

Acknowledgments Teresa Mata thanks Fundac¸ a˜o para a Cieˆncia e Tecnologia for their support through provision of a postgraduate scholarship.

Supporting Information Available EPA’s WAR algorithm, details on compositions of gasoline blends and their effects on the various impact categories, and detailed descriptions of Figures 4 and 5. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review January 9, 2003. Revised manuscript received May 22, 2003. Accepted May 29, 2003. ES034024S